Recent work has shown that activation of the alternative pathway of complement is central to the pathogenesis of a large number of glomerular and tubulointerstitial renal diseases. It is not known, however, why the kidney is so frequently and uniquely vulnerable to alternative pathway-mediated injury. The central hypothesis of this grant is that the plasma protein factor H is critical for controlling alternativ pathway activation on the glomerular basement membrane and the surface of injured mesangial cells and tubular epithelial cells. Complement regulation within the kidney by factor H is disrupted by the local production of proteins that impair factor H function and by complement-activating microparticles shed by injured cells throughout the body. To test this hypothesis, the following specific aims will be pursued. Aim 1) Determine which regulatory proteins control complement activation in the kidney. The working hypothesis for this aim is that factor H prevents spontaneous complement activation on the GBM and on injured mesangial and tubular epithelial cells. We will use in vitro and in vivo methods to determine which renal surfaces require factor H to prevent complement-mediated injury. We will use targeted complement inhibitors to restore factor H to sites of complement activation within the kidney. Aim 2) Determine how the protein annexin A2 modulates complement regulation by factor H within the kidney. The working hypothesis for this aim is that annexin A2 expressed within the injured kidney blocks factor H function, thus permitting local activation of the alternative pathway. The experiments in this aim will utilize recombinant annexin A2, SiRNA, and mice with targeted deletion of the gene for annexin A2 in order to determine how annexin A2 promotes complement activation on the surface of cells of the kidney in vitro and in vivo. Aim 3) Determine whether plasma microparticles impair complement regulation within the kidney. The working hypothesis of this aim is that microparticles shed from injured endothelial cells cause complement activation within the kidney. These experiments will utilize cyclosporine to generate endothelial microparticles and test whether they cause injury in factor H deficient mice. Cyclosporine-induced microparticles will be injected into factor H deficient mice to determine whether the microparticles directly increase alternative pathway activation and cause renal injury. Finally, we will test whether CR2-factor H, a targeted complement inhibitor, prevents cyclosporine-induced vascular and renal toxicity in factor H deficient mice. The approach outlined above is innovative because it challenges the current understanding of how factor H controls complement activation on tissues and it provides an explanation for why the kidney, among all of the organs, is so uniquely susceptible to alternative pathway-mediated injury. The studies in this grant are significant because they examine a process central to the pathogenesis of a wide range of renal diseases and provide new treatment strategies for these diseases.